It is well known that the vortex formed at the tip of aircraft wings and blades moving relatively through the air may give rise to some undesirable resulting effects. One such effect is the resultant increase in drag on the wing caused by the tip vortex. Particular attention has been paid to this effect with respect to the wing surfaces of fixed-wing aircraft, and numerous wing tip designs have been proposed or used to minimize that vortex-induced drag penalty in such aircraft. One such design is described in U.S. Pat. No. 5,102,068 (hereinafter '068) to Gratzer for Spiroid-Tipped Wing.
In that '068 patent, a spiroidal tip device of particular geometry and orientation, depicted in Prior Art FIGS. 1A-1C, is used principally to reduce induced drag. A typical wing end portion 1 has a spiroid 2 attached thereto. The spiroid 2 joins the wing at a sweep angle .DELTA., measured relative to the wing tip chord line 4 and included angle .phi., measured relative to the projected wing plane 3. The subscripts F and A are used to designate the forward and aft ends respectively. The spiroid surface cross sections are typical airfoil sections 5, shown inset in FIG. 1A. The spiroid chord, which is a function of distance along a spiroidal generator 6, decreases approximately linearly from slightly more than half the wing tip chord at its forward end to about 0.3 of the wing tip chord at the spiroid midpoint; thereafter it increases approximately linearly to about half the wing tip chord at its aft end.
Another undesirable effect of such tip vortices is the noise, typically audible, created when the vortex interacts with an object, such as another trailing blade or the body and empennage structures of an aircraft. This phenomenon is known as blade vortex interaction, or simply, "BVI". The resulting noise may be particularly objectionable with respect to the operation of rotorcraft, such as helicopters, tilt-rotors, and other rotary-wing aircraft, especially during low speed descent to landing and maneuvers. It is this latter effect (i.e.,BVI) that is to be addressed by the present invention.
While the aforementioned '068 Patent also suggests applicability of its spiroid-tipped wing to reduce noise and that it may be used with helicopters, its geometry and orientation are relatively complex and not well suited for use under the high centrifugal loadings imposed by rotary-wing aircraft. Indeed, the spiroid-tipped wing of that patent discloses only an arrangement that is highly asymmetrical to the loadings normally occasioned at the tip of a rotary-wing blade, as noted by the placement of spiroid 2 entirely above the extended plane 3 of wing 1. Moreover, its geometry is sufficiently varied over the extent of the spiroid as to make it difficult to manufacture.
It is thus an object of the present invention to provide an improved rotary-wing blade tip that reduces blade-tip vortex energy intensity to thereby reduce noise resulting from subsequent vortex interactions. It is a further object of the invention to provide such an improved rotary-wing blade tip in a manner that minimizes additional loading and stresses on the rotary-wing blade during operation.